Concept:Nitrogen-fixing bacteria are associated with the root nodules of leguminous plants.Explanation:Leguminous plants develop nodules on their roots.These root nodules contain nitrogen-fixing bacteria such as Rhizobium.Azotobacter is a free-living nitrogen-fixing bacterium, but among the given parts of a leguminous plant, the bacteria are best located in the root nodules.Therefore, the correct part is the root nodules.Answer:C. in the root nodules